Torque converters exist which are capable of switching between three operation states, a converter state, a lockup state, and a slip state.
The converter state is a state in which input elements and output elements are completely released, and torque is transmitted through a fluid. The lockup state is a state that is selected in an operating region where torque increasing actions and transmission shock absorption functions are not necessary, in order to reduce degradation of fuel efficiency caused by slippage of the torque converter. The input elements of the torque converter are directly connected to the output elements in this state. The slip state maintains a state in which slip develops between the input elements and the output elements with the lockup clutch in a half connected state.
In JP2665597B, when a lockup differential pressure increases such that a torque converter reaches a slip state from a converter state, the maximum amount for the lockup differential pressure is set based on a slip rotational speed ΔN (ΔN=a turbine runner rotational speed Nt−an engine rotational speed Ne) when control begins.
Among parameters that determine the slip rotational speed (the turbine runner rotational speed Nt and the engine rotational speed Ne), the turbine runner rotational speed (transmission input rotational speed) depends upon running resistance and grade resistance. When comparing running along a flat roadway and running along a climbing roadway, for example, the turbine runner rotational speed increases more slowly for the climbing roadway than for the flat roadway.
For a continuously variable transmission, the turbine runner rotational speed is a value that is determined according to vehicle speed. Accordingly, the rate of increase in the turbine runner rotational speed is determined by the rate of increase in the vehicle speed during a period until reaching a point where speed changes begin. Referring to FIG. 22, when acceleration is effected from a vehicle speed VSP=0 with a throttle opening TVO=3/8, speed changes begin from a point where the vehicle speed corresponding to a point A in the drawing is reached, toward a Hi side. The gear change ratio remains at a least Low value in a period up to that point.
However, the conventional technique described above does not take into consideration the fact that the turbine runner rotational speed depends upon the running resistance, the grade resistance, and the like during the period until speed changes begin. The amount of increase in the lockup differential pressure is thus set based only on information at the start of control, and torque converter transition is effected from the converter state to the slip state under open loop control.
For example, the same amount of increase is set when running along a flat roadway and when running along a climbing roadway. Accordingly, referring to FIG. 16A, the torque converter transition from the converter state to the slip state takes place after the vehicle speed has sufficiently increased when running a flat roadway. However, referring to FIG. 16B, the torque converter transition to the slip state takes place before the vehicle speed increases. The torque converter transition to the slip state is complete before open loop control ends at a low turbine runner rotational speed. Accordingly, muffled sounds and vibrations tend to develop, and further, the torque converter transition to the slip state takes place at a turbine runner rotational speed that is lower than normal. A stepwise change in the engine rotational speed when transitioning to the slip state thus becomes larger.